This application is based upon and claims benefit of priority of Japanese Patent Application No. 2000-319266 filed on Oct. 19, 2000, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a starter motor for cranking an internal combustion engine, the starter motor including a labyrinth-shaped ventilation passage.
2. Description of Related Art
An example of a starter motor having a ventilation passage is disclosed in JP-A-7-103116. A relevant portion of the starter motor is illustrated in FIG. 6. Cavities 110 are formed in a clutch case 100, an inner space of the starter motor (a motor chamber) communicates with outside through the cavities 110 and a clutch chamber 120. In this manner, a ventilation passage connecting the inner space of the starter motor to the outside can be made long and complex, and accordingly it is possible to prevent outside water from entering into the inner space of the starter motor through the ventilation passage.
In the conventional motor, however, a communicating hole 130 connecting the cavities 110 to the clutch chamber 120 is positioned at an outer peripheral portion of a reduction gear 140 that is housed in an upper portion of the clutch chamber 120. Therefore, there is a problem that the communicating hole 130 is closed by lubricating grease scattered from the rotating gear 140. Moreover, brush dusts generated in the motor chamber enter into the clutch chamber 120 through the ventilation passage and mixes with the grease. The reduction gear 140 and associated parts are abnormally abraded by the brush dusts mixed with the grease. Further, there is a possibility that the brush dusts contained in ventilation air adhere to and close the ventilating hole 130.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved ventilation passage in the starter motor, so that the ventilation passage is not clogged with the scattered grease, and the brush dusts generated in the motor chamber is prevented from being scattered in the ventilation passage.
The starter motor is composed of a motor for generating a rotational torque for cranking an internal combustion engine, a magnetic switch for supplying electric power from a battery to the motor, and a reduction gear train for transferring a rotational torque of the motor to the engine. An inside space of the starter motor is ventilated by air flowing through a ventilation passage that is open to outside of the starter motor. The ventilation passage includes a communicating passage connecting a switch chamber containing the magnetic switch therein and a motor chamber containing the motor therein.
The communicating passage is formed along a boundary surface between a front housing containing the reduction gear train and a center housing having the motor space and the switch space. Cavities formed in the front housing and cavities formed in the center housing are positioned to partially overlap one another when both housings are connected. The cavities overlapped in this manner form a labyrinth passage. The communicating passage that includes the labyrinth passage is formed around the reduction gear train, and the gear train is separated from the communicating passage by a separating wall.
When the motor rotates upon actuation of the magnet switch, pressure inside the motor chamber and switch chamber vibrates, and the inside air is heated. The heated air is ventilated through the ventilation passage. The labyrinth passage formed in the communicating passage prevents brush dusts generated in the motor chamber from being scattered in the ventilation passage and being mixed with grease lubricating the reduction gear train. Further, since one of the cavities constituting the labyrinth passage is positioned at the bottom of the front housing, the brush dusts are kept therein. Since the ventilation passage is separated from the reduction gear train, the grease splash from the gear train does not enter into the ventilation passage.
The ventilation passage according to the present invention is kept free from the grease and the brush dusts without being clogged therewith. The inside space of the starter motor is well ventilated by the air flowing through the ventilation passage.